This invention relates to a transmission amplifier and, more particularly, to a transmission amplifier, which is equipped with a distortion compensating function, so adapted as to be capable of both stand-alone operation and parallel operation.
In wireless communications in recent years, there is growing use of high-efficiency transmission using digital techniques. In instances where multilevel amplitude modulation is applied to wireless communications, a vital technique is one which can suppress non-linear distortion by linearizing the amplification characteristic of the power amplifier on the transmitting side and reduce the leakage of power between adjacent channels. Also essential is a technique which compensates for the occurrence of distortion that arises when an attempt is made to improve power efficiency by using an amplifier that exhibits poor linearity.
FIG. 20 is a block diagram illustrating an example of a transmitting apparatus in a radio according to the prior art. Here a transmit-signal generator 1 transmits a serial digital data sequence and a serial/parallel (S/P) converter 2 splits the digital data sequence alternately one bit at a time to convert the data to two sequences, namely an in-phase component signal (also referred to as an xe2x80x9cI signalxe2x80x9d) and a quadrature component signal (also referred to as a xe2x80x9cQ signalxe2x80x9d). A DA converter 3 converts the I and Q signals to respective analog baseband signals and inputs these to a quadrature modulator 4. The latter multiplies the input I and Q signals (the transmit baseband signals) by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90xc2x7 and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. A frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and a transmission power amplifier 6 power-amplifies the carrier output from the frequency converter 5. The amplified signal is released into space from an antenna 7.
In mobile communications based upon W-CDMA, etc., the transmission power of the transmitting apparatus is a high ten watts to several tens of watts, and the input/output characteristic [distortion function f(p)] of the transmission power amplifier 6 is non-linear, as indicated by the dotted line in (a) of FIG. 21. Non-linear distortion arises as a result of this non-linear characteristic, and the frequency spectrum in the vicinity of a transmission frequency f0 develops side lobes, as shown in (b) of FIG. 21, leakage into the adjacent channel occurs and this causes interference between adjacent channels. More specifically, owing to non-linear distortion, there is an increase in power that causes transmitted waves to leak into the adjacent frequency channel, as shown at (b). ACPR (Adjacent Channel Power Ratio), which indicates the magnitude of leakage power, is the ratio between the power of the channel of interest, which is the area of the spectrum between the one-dot chain lines A and Axe2x80x2 in FIG. 21(b), and the adjacent leakage power, which is the area of the spectrum between the two-dot chain lines B and Bxe2x80x2, that leaks into the adjacent channel. Such leakage power constitutes noise in other channels and degrades the quality of communication of these channels. Such leakage must be limited to the utmost degree.
Leakage power is small in the linear region [see (a) in FIG. 21] of the power amplifier and large in the non-linear region. Accordingly, it is necessary to broaden the linear region in order to obtain a transmission power amplifier having a high output. However, this necessitates an amplifier having a performance higher than that actually needed and therefore is inconvenient in terms of cost and apparatus size. Accordingly, a transmission apparatus that has come to be adopted is equipped with a distortion compensating function that compensates for distortion ascribable to non-linearity of the power amplifier.
FIG. 22 is a block diagram of a transmitting apparatus having a digital non-linear distortion compensating function that employs a DSP (Digital Signal Processor). Here digital data (a transmit signal) sent from the transmit-signal generator 1 is converted to the two sequences of I and Q signals by the S/P converter 2. These signals enter a distortion compensator 8 constituted by a DSP. The distortion compensator 8 includes a distortion compensation coefficient memory 8a for storing distortion compensation coefficients h(pi) (i=0xcx9c1023) conforming to power levels pi of a transmit signal x(t); a predistortion unit 8b for subjecting the transmit signal to distortion compensation processing (predistortion) using a distortion compensation coefficient h(pi) that is in conformity with the power level of the transmit signal; and a distortion compensation coefficient calculation unit 8c for comparing the transmit signal x(t) with a demodulated signal (feedback signal) y(t), which has been obtained by demodulation in a quadrature detector described later, and for calculating and updating the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will approach zero.
The transmit signal that has been subjected to predistortion processing by the distortion compensator is input to the DA converter 3. The latter converts the input I and Q signals to analog baseband signals and applies the baseband signals to the quadrature modulator 4. The latter multiplies the input I and Q signals by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90xc2x7 and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. The frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and the transmission power amplifier 6 power-amplifies the carrier signal that is output from the frequency converter 5. The amplified signal is released into space from the antenna 7.
Part of the transmit signal is input to a frequency converter 10 via a directional coupler 9 so as to undergo a frequency conversion and then be input to a quadrature detector 11. The latter multiplies the input signal by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90xc2x0 to thereby perform quadrature detection, reproduces the I, Q signals of the baseband on the transmitting side and applies these signals to an AD converter 12. The latter converts the applied I and Q signals to digital data and inputs the digital data to the distortion compensator 8. By way of adaptive signal processing using the LMS (Least Mean Square) algorithm, the distortion compensator 8 compares the transmit signal before the distortion compensation thereof with the feedback signal demodulated by the quadrature detector 11 and proceeds to calculate and update the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will become zero. By subsequently repeating this operation, non-linear distortion of the transmission power amplifier 6 is suppressed to reduce the leakage of power between adjacent channels.
FIG. 23 is a diagram useful in describing distortion compensation processing by an adaptive LMS. A multiplier 15a (which corresponds to the predistortion unit 8b in FIG. 22) multiplies the transmit signal x(t) by a distortion compensation coefficient hn(p). A DA converter 15b converts the distortion-compensated signal to an analog signal, which is applied to a power amplifier 15c having a distortion function f(p). A feedback loop 15d feeds back the output signal y(t) from the power amplifier and digital converter 15e converts the analog feedback signal to a digital signal. A power calculation unit 15f calculates the power p [=x(t)2] of the transmit signal x(t) and outputs the power as a read-in address of a distortion compensation coefficient memory 15g. The memory 15g (which corresponds to the distortion compensation coefficient memory 8a of FIG. 22) stores the distortion compensation coefficients that conform to the power levels of the transmit signal x(t). The memory 15g outputs the distortion compensation coefficient hn(p) conforming to the power p of the transmit signal x(t) and updates the distortion compensation coefficient hn(p) by a distortion compensation coefficient hn+1(p) found by the LMS algorithm.
A distortion coefficient calculation unit 15h calculates the distortion compensation coefficient hn+1(p) found by the LMS algorithm. A delay circuit 15i is for generating a write address, a delay circuit 15j adjusts the timing at which the distortion coefficient hn(p) is output, and the delay circuit 15k adjusts the timings of the transmit signal x(t) and feedback signal y(t). The delay circuit 15k adds a delay time TD, which lasts from arrival of the transmit signal x(t) to input of the feedback signal y(t) to a multiplier 21, to the transmit signal X(t). A delay time decision unit 15m decides the delay time between the transmit signal x(t) and the feedback signal by a correlation operation. It should be noted that the transmit signal processor of FIG. 23 also includes a modulator/demodulator and a frequency converter, though these are not shown.
The distortion coefficient calculation unit 15h includes a subtractor 21 that outputs the difference e(t) between the transmit signal x(t) prior to distortion compensation and the feedback signal y(t); a multiplier 22 that performs multiplication between the error e(t) and a step-size parameter xcexc; a complex-conjugate signal output unit 23 for outputting a complex-conjugate signal y*(t); a multiplier 24 for multiplying hn(p) by y*(t); a multiplier 25 for multiplying xcexce(t) by u*(t); and an adder 26 for adding the distortion compensation coefficient hn(p) and xcexce(t)u*(t). The arithmetic operations performed by the arrangement set forth above are as follows:
hn+1(p)=hn(p)+xcexce(t)u*(t)
e(t)=x(t)xe2x88x92y(t)
y(t)=hn(p)x(t)f(p)
u(t)=x(t)f(p)=hn(p)y*(t)
p=|x(t)|2
where x, y, f, h, u, e represent complex numbers and signifies a complex conjugate. By executing the processing set forth above, the distortion compensation coefficient h(p) is updated so as to minimize the difference signal e(t) between the transmit signal x(t) and the feedback signal y(t), and the coefficient eventually converges to the optimum distortion compensation coefficient value so that compensation is made for the distortion in the transmission power amplifier.
The delay time decision unit 15m calculates correlation y(t) between the transmit signal x(t) prior to distortion compensation and the feedback signal, decides total delay time D (=D0+D1), which is produced by the power amplifier 15c and feedback loop 15d, etc., based upon the maximum correlation, and sets the delay in the delay circuits 15i, 15j, 15k. In the calculation of correlation, the delay time decision unit 15m shifts the delay time between the transmit signal x(t) and feedback signal y(t) successively a predetermined length of time, calculates the correlation between the transmit signal and feedback signal at each delay time and determines the delay time D (=D0+D1) for which correlation is maximized. In actuality, the determination of delay time is performed over a plurality of steps. For example, at a first step, as shown in FIG. 24, a search range is made a maximum delay time TA, and a time period xcex94TA of the delay-time search is enlarged to find a rough delay-time range TB. Next, at a second step, the search range is made a delay-time range TB, and a time period xcex94TB of the delay-time search is reduced (xcex94TB less than xcex94TA) to find a delay-time range TC. Thereafter, and in similar fashion, the time period of the search is reduced to narrow down the delay-time search range. At a final step, a search range TD is made several hundred nanoseconds and the time period xcex94TD of the delay-time search is made several tens of picoseconds (one sampling clock), whereby a highly precise delay time is decided. If this delay time cannot be set correctly, the distortion compensating function will not be performed effectively. The larger the delay-time setting error, the smaller the degree of distortion compensation, the larger the side lobes and the greater the leakage of power to the adjacent channel.
The components from the distortion compensator to the power amplifier shall be referred to as a transmission amplifier. The transmission power of this transmission power amplifier presently is limited to 40 to 50 W. If power is less than 40 to 50 W, the transmission power amplifier operates in stand-alone fashion; if power is greater than 40 to 50 W, e.g., 80 W, the transmission power amplifier operates in parallel fashion. The transmission amplifier therefore is so adapted as to be capable of stand-alone operation, in which the amplifier output signal emanates from the antenna as is, and parallel operation, in which the output signals of two amplifiers are combined before being released from the antenna. The stand-alone operation often is used in a transmission diversity operation, and the parallel operation often is used in a non-diversity operation.
FIG. 25 is a diagram showing a transmission amplifier in a stand-along configuration, and FIG. 26 is a diagram showing transmission amplifiers in a parallel-operation configuration. A transmission amplifier 30 comprises a transmit-signal processor 31 and an amplifier 32 and is housed within a case 33. The transmit-signal processor 31 has a distortion compensator, a DA/AD converter, a quadrature modulator/demodulator and a frequency converter (up-converter/down-converter), etc., though these are not shown. The case 33 is provided with a first terminal T1 to which the transmit signal x(t) is input, a second terminal T2 for sending the signal that is output from the transmit-signal processor 31, a third terminal T3 to which the signal input to the amplifier 32 is applied, a fourth terminal T4 from which the output signal of the amplifier 32 is sent, and a fifth terminal T5 to which a feedback signal is input.
At the time of stand-alone operation, the second and third terminals are directly connected and the fourth and fifth terminals are directly connected, as shown in FIG. 25. At the time of parallel operation, on the other hand, as shown in FIG. 26, second terminals T2, T2 of transmission amplifiers 30A, 30B are connected to two input terminals of a hybrid HYB (distributor/combiner) 33, the output terminal of the hybrid HYB 33 is connected to the input terminal of a hybrid HYB 34, and two output terminals of the hybrid HYB 34 are connected to third terminals T3, T3 of transmission amplifiers 30A, 30B. Further, two input terminals of a hybrid HYB 35 are connected to amplifier output terminals T4, T4 of the transmission amplifiers 30A, 30B, and the output terminal of the hybrid HYB 35 is connected to fifth terminals T5, T5 of the transmission amplifiers 30A, 30B. At the time of parallel operation, the transmit-signal processor 31 of only one of the transmission amplifiers 30A, 30B is operated and the operation of the other transmit-signal processor is halted. If the transmit-signal processor 31 of the transmission amplifier 30A is operated, for example, the signal output from the transmit-signal processor 31 is input to the hybrid 34 via the hybrid 33, and the hybrid 34 branches the signal to the amplifiers 32, 32 of the transmission amplifiers 30A, 30B. The transmit signals that have been amplified by the amplifiers 32 of the transmission amplifiers 30A, 30B are input to the hybrid HYB 35, where the signals are combined and released from the antenna and fed back to the fifth terminal T5 of the transmission amplifier 30A.
Whether operation is stand-alone or parallel depends upon whether hybrids (distributor/combiner) are installed on the shelf that accommodates the transmission amplifiers. At the time of manufacture, therefore, it is not known which mode of operation will be adopted for the transmission amplifiers. Owing to this system of operation, the following problems arise when the transmission amplifier is started up:
When each transmission amplifier is manufactured, the delay time of the feedback signal that prevails at the time of stand-alone operation is measured and then recorded internally. If stand-alone operation is adopted in the field, then the recorded delay time is read out and set in the delay circuits 15i, 15j, 15k (see FIG. 23), thereby making high-speed start-up possible. If parallel operation is adopted, however, the delay time will differ from that of stand-alone operation and a delay adjustment (measurement of delay time and setting thereof) must be made again starting from the first step of FIG. 24. Furthermore, measuring and recording delay time of the feedback signal, which will prevail at the time of parallel operation, when the amplifier is manufactured is difficult from the standpoints of improving ease of manufacture and avoiding an increase in factory equipment.
Another problem is that if the adjustment of feedback-signal delay is performed from the first step, several minutes to ten-odd minutes will be required and it will take considerable time before the radio waves can be emitted.
A further problem is that when transmission power varies between delay adjustments, the accuracy of the delay adjustment declines. Since delay adjustment time is a long several minutes to ten-odd minutes, a change in transmission power occurs frequently.
Yet another problem is that since distortion compensation is not carried out between delay adjustments, unnecessary waves (distortion) are transmitted when output power is above a certain value.
Accordingly, an object of the present invention is to make possible high-speed start up equivalent to that at the time of stand-alone operation even at the time of parallel operation.
Another object of the present invention is to make high-speed start-up possible by deciding delay time in a short time (several seconds or less) even in a case where a delay adjustment is necessary in order to accommodate for a variance in amount of delay in a passive circuit such as a hybrid mounted externally of a transmission amplifier.
A further object of the present invention is to prevent a decline in delay-time decision accuracy even in a case where transmission power before and transmission power after determination of delay time differs by more than a fixed value.
Another object of the present invention is to contribute to maintenance of communication stability and quality of radio waves by preventing the transmission of unnecessary waves when delay time is decided.
A further object of the present invention is to return to a normal state of communication rapidly after a delay-time decision while suppressing transmission of unnecessary waves.
According to the present invention, the foregoing objects are attained by providing a transmission amplifier having a distortion compensator for compensating for distortion of an amplifier by updating distortion compensation coefficients so as to null a difference between a transmit signal and a feedback signal, and subjecting the transmit signal to distortion compensation processing using the distortion compensation coefficients; an amplifier for amplifying the transmit signal that has undergone distortion compensation; a feedback unit for feeding an output signal from the amplifier back to an arithmetic unit that; calculates the difference in the distortion compensator; and a delay circuit for inputting the transmit signal to the arithmetic unit upon delaying the transmit signal for a delay time equivalent to time required for the feedback signal to arrive at the arithmetic unit. In order to set an accurate delay time in the delay circuit at the time of stand-alone operation and parallel operation, the transmission amplifier of the present invention includes a storing unit for storing, in advance, a delay time TDS of the feedback signal at the time of stand-alone operation and a delay time difference xcex94T [or the sum (TDS+xcex94T)] between delay times of the feedback signal at the time of stand-alone operation and at the time of parallel operation; and a delay-time setting unit for setting the delay time TDS in the delay circuit at the time of stand-alone operation and setting (TDS+xcex94T) in the delay circuit at the time of parallel operation. By thus providing the transmission amplifier with the storing unit and delay-time setting unit, high-speed start-up similar to that at the time of stand-alone operation becomes possible.
Further, by providing a delay-time decision unit for deciding an accurate delay time by adopting a narrow range in the vicinity of the set delay time as the object of a search, it is possible to achieve high-speed start-up by deciding delay time in a short time (several seconds or less) even in a case where a delay adjustment is necessary.
Further, by providing a power measurement unit for measuring transmit-signal power and a controller for performing control so as to decide delay time again when transmission power before and transmission power after a delay-time decision differs by more than a fixed value, a decline in the precision of the delay-time decision can be prevented.
Further, by providing a gain varying unit for controlling gain of the transmit signal and a controller for controlling the gain varying unit, when delay time is decided, to lower the gain of the transmit signal and then restore the original gain of the transmit signal after delay time is decided, the transmission of unnecessary waves when delay time is decided is prevented, thereby making it possible to contribute to maintenance of communication stability and quality of radio waves. In this case, by gradually restoring the lowered gain to the original gain following the delay-time decision, distortion compensation can be performed stably and a normal state of communication can be restored rapidly while transmission of unnecessary waves is suppressed.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.